1. Field of the Invention
This invention relates to recovery of reaction products (i.e. monomers and other depolymerization products) from contaminated plastics. Specifically, this invention relates to processes for recovering monomers from polymers having a desired polymer content below about 98%.
2. Technical Background of the Invention
The inadequate treatment of waste which is landfilled, and the increasing percentage of nondegradable materials, including plastics, in municipal solid waste streams, have increased the cost of solid waste disposal and further stimulated public pressure to recycle nondegradable plastic material.
Over the years there have been many technological developments in the field of production and use of polymers. Various additives, modifiers, comonomers, copolymers, and fillers have been incorporated into polymers to improve characteristics such as strength and temperature resistance, and to thereby meet the needs of more specialized applications. Polymers have also been used in conjunction with other materials to make complex systems and composites where separation of the individual materials would be difficult. In addition to material added in the manufactured polymer, post-consumer solid waste usually contains contamination introduced during consumer use of the article or during the collection process. The presence of these contaminants, and materials incorporated during manufacture, have limited the effectiveness of post-consumer plastic recycling. The problem is one of initial low purity of the desired plastic and the necessity to process a wide range of other materials that may be present.
Polyesters and polyamides may be recycled by various methods to yield useful polymers, oligomers and monomers. Traditional chemical recovery techniques include hydrolysis, glycolysis and methanolysis for polyesters, and hydrolysis and ammonolysis for polyamides. For polyesters, these methods are most often combined with an initial depolymerization step, which is accomplished by heating and/or dissolving the polymer in oligomers, monomers (such as ethylene glycol), or water.
Hydrolysis involves treating the starting polymer with water and heat. Complete depolymerization will yield monomers (e.g., terephthalic acid (TPA) and ethylene glycol (EG) for polyethylene terephthalate (PET); and hexamethylene diamine (HMD) and adipic acid for nylon 6'6), which can then be polymerized. For PET, additional additives such as salts, NaOH, H.sub.2 SO.sub.4, and NH.sub.3 OH, are sometimes used to enhance the process. See U.S. Pat. Nos . 4,355,175, 3,544,622, 3,952,053 and 4,542,239, respectively. Additionally, hydrolysis, specifically steam treatment, can be used in conjunction with other treatments discussed below, see U.S. Pat. No. 3,321,510.
Another recovery method for PET, glycolysis, is accomplished by using a glycol, e.g. ethylene glycol (EG) or butane diol (BDO), to break down the polymer. This has been done in the liquid phase, and usually employs heat and pressure. Glycolysis of PET with ethylene glycol yields bis-.beta.-hydroxyethyl terephthalate (BHET) which is then usually filtered to remove impurities and polymerized, see U.S. Pat. No. 4,609,680. Glycolysis can be combined with a second step-like methanolysis, see U.S. Pat. No. 3,321,510.
The third method for polyesters, alcoholysis, e.g., methanolysis, breaks down the polymer back to its monomers. Conventional methanolysis generally operates using a polymer melt in which super heated methanol is bubbled through the mixture. See for example EPO Patent Application 0484963A3 and U.S. Pat. No. 5,051,528. Methanolysis can optionally include the use of catalysts to enhance the recovery rate, see, for example, U.S. Pat. Nos. 3,776,945 and 3,037,050, as well as the use of organic solvents, see U.S. Pat. No. 2,884,443. Methanolysis can be used in conjunction with various initial depolymerization methods, for example, dissolving the polymer in its oligomers, see U.S. Pat. No. 5,051,528; depolymerizing using EG, see Japanese Patent No. 58,020,951 B4; or depolymerizing using water, see U.S. Pat. No. 3,321,510. After alcoholysis of PET with methanol and recovering the monomers, an additional refining step may be used to separate and purify the DMT from EG. This can be done by precipitation, distillation, or cystallization.
For polyamides, ammonolysis can be used to break down the polymer back to monomers. For example, Japanese Patent Application Publication 54-84,525 (1979) describes a process to obtain the monomers 6-aminocapronitrile (6ACN) and caprolactam (CL) which is accomplished by treating molten polycaproamide (nylon 6) at elevated temperature and pressure with ammonia gas. British Patent 1,172,997 discloses the conversion of a polyamide into monomeric compounds by heating the polyamide (nylon 6 and nylon 6,6) with ammonia in the presence of hydrogen and a hydrogenation catalyst. With nylon 6,6 the monomers obtained are hexamethylene diamine (HMD) and hexamethyleneimine and a small amount of unidentified material. With nylon 6, the monomers obtained are HMD, hexamethyleneimine and N-(6-aminohexyl)-hexamethyleneimine.